Problems with GtPlot

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I´m working in the sector "P-recovery from sewage". With GWB I want to reproduce what´s happening with different sewage. The treatment of supernatant liquor shows struvite as recovered product. So I expanded thermo.dat by the mineral struvite. After several complications ( ) it worked. But now: If I choose in the basis "HPO4--" I just get Hydroxylapatite in GtPlot (while Hydroxylapatite and Struvite are supersaturated". If I choose "PO4---" I just get Struvite. The pH-value is neutral to alkaline, so "HPO4--" has to be the right form....Another problem: I just have the thermodynamic constant for Struvite at 25 °C, no other one.

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I've looked at your database- there's a problem with the way you've defined your Basis species- you have both HPO4-- and PO4--- as Basis species. Have a look at Section 3.2 (p. 36) of Craig Bethke's "Geochemical and Biogeochemical Reaction Modeling" text. Regarding your choice of Basis components, from the text:

"...the components must be linearly independent of one another. In other words, we should not be able to write a balanced reaction to form one component in terms of the others"

In your case, HPO4-- can be formed from H+ and PO4---. The reaction for Struvite in your database is written in terms of the PO4--- basis species, while Hydroxyapatite is written in terms of the other phosphate basis species, HPO4--. Hence, you get the spurious results that you do. I recommend choosing one phosphate basis species (HPO4-- is the default for thermo.dat), and making the other an aqueous species.

As far as having only data at 25C, this is not a problem since your model is at 25C (although the dataset you sent me appears to have constants at other temperatures?)

I hope that helps,

Tom Meuzelaar

RockWare, Inc.

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Tom - I understood your statement. It seems to be logical. But: I doesn´t work. I tried this day several variants. But always I get error messages. Either unexplainable (error in line 351 Zn+) oder "unknown reaction entry HPO4 oder PO4". It only works as I mentioned (HAp but no struvite, also by suppressing HAp - no struvite in the plot) if I choose HPO4-- AND PO4--- in the basis. I also cancelled all minerals having "HPO4" in their reaction entry to avoid the error message "unknown reaction entry"....

I´m sorry for being so problematical.

Wish you a nice weekend!

Yours, Anke

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Tom - I understood your statement. It seems to be logical. But: I doesn´t work. I tried this day several variants. But always I get error messages. Either unexplainable (error in line 351 Zn+) oder "unknown reaction entry HPO4 oder PO4". It only works as I mentioned (HAp but no struvite, also by suppressing HAp - no struvite in the plot) if I choose HPO4-- AND PO4--- in the basis. I also cancelled all minerals having "HPO4" in their reaction entry to avoid the error message "unknown reaction entry"....

I´m sorry for being so problematical.

Wish you a nice weekend!

Yours, Anke

Hi Anke:

No worries- GWB involves a long and on-going learning curve for all of us, myself included.

Can you send me the database with Struvite and with only one Basis entry for phosphate (HPO4-) so I can reproduce the error message you are getting?